The outer layer of skin surrounding the body performs an important protective function as a barrier against infection, and serves as a means of regulating the exchange of heat, fluid and gas between the body and external environment. When skin is removed or damaged by being abraded, burned or lacerated, this protective function is diminished. Areas of damaged skin are conventionally protected by the application of a wound dressing which facilitates wound healing by acting as a skin substitute.
Wounds to skin and the underlying tissues of animals may be caused by external insult such as friction, abrasion, laceration, burning or chemical irritation. Damage to such tissues may also result from internal metabolic or physical dysfunction, including but not limited to bone protrudence, diabetes, circulatory insufficiencies, or inflammatory processes. Normally tissue damage initiates physiological processes of regeneration and repair. In broad terms, this process is referred to as the wound healing process.
The wound healing process usually progresses through distinct stages leading to the eventual closure, and restoration of the natural function of the tissues. Injury to the skin initiates an immediate vascular response characterized by a transient period of vasoconstriction, followed by a more prolonged period of vasodilation. Blood components infiltrate the wound site, endothelial cells are released, exposing fibrillar collagen, and platelets attach to exposed sites. As platelets become activated, components are released which initiate events of the intrinsic coagulation pathway. At the same time, a complex series of events trigger the inflammatory pathways generating soluble mediators to direct subsequent stages of the healing process.
Normally, the wound healing process is uneventful and may occur regardless of any intervention, even in the case of acute or traumatic wounds. However, where an underlying metabolic condition or perpetual insult such as pressure is a contributing factor, the natural wound healing process may be retarded or completely arrested, resulting in a chronic wound. Trends in modern medical practices have shown that the wound healing of both acute and chronic wounds may be significantly improved by clinical intervention using methods and materials that optimize wound conditions to support the physiological processes of the progressive stages of wound healing. Key factors in providing the optimal conditions are the prevention of scab formation and the maintenance of an optimal level of moisture in the wound bed. Both of these factors can be controlled by the management of wound exudate fluid.
A common problem in the management of both acute and chronic wounds is the maintenance of an optimal level of moisture over the wound bed during heavy exudate drainage. This is usually, but not always, an early stage of healing. Most moist wound dressing technologies such as thin films, hydrocolloid dressings and hydrogels are typically overwhelmed by the accumulated exudate moisture during this heavy drainage phase. Management of moisture during heavy exudate drainage often necessitates the use of gauze or sponge packings that wick away excess moisture from the wound bed, thin film coverings that trap exudate fluid over the wound bed, or calcium alginate dressings that chemically bind exudate moisture due to the hydroscopic properties of the seaweed extract.
Examples of wound dressings that have been developed include collagen dressings. Soluble collagen has been used as a subcutaneous implant for repairing dermatological defects such as acne scars, glabellar furrows, excision scars and other soft tissue defects. Collagen has also been used in many forms as wound dressings such as collagen sponges, as described in Artandi, U.S. Pat. No. 3,157,524 and Berg et al, U.S. Pat. No. 4,320,201. However, most of these dressings are not satisfactory for the various types of full thickness wounds. Collagen films and sponges do not readily conform to varied wound shapes. Furthermore, some collagen wound dressings have poor fluid absorption properties and undesirably enhance the pooling of wound fluids.
Another example of wound dressings that have been developed are hydrocolloid dressings. UK Patent Number 1,471,013 and Catania et al., U.S. Pat. No. 3,969,498 describe hydrocolloid dressings that are plasma soluble, form an artificial eschar with the moist elements at the wound site, and gradually dissolve to release medicaments. These dressings comprise a hydrophilic foam of dextran polymer that can be applied without therapeutic agents or ointments, are non-irritating to the lesion and can be easily removed.
Known hydrocolloid dressings in general, and the Catania et al. dressings in particular, are subject to a number of drawbacks. The major disadvantages of these dressings include the potential to disintegrate in the presence of excess fluid at the wound site, and minimal (virtually negligible) control over water loss from the wound. This latter disadvantage is particularly important as excess water loss from a wound will cause an increase in heat loss from the body as a whole, potentially leading to hypermetabolism. In addition, hydrocolloid dressings require frequent dressing changes. This is especially true of the Catania et al. dressing due to the dissolution of the dextran polymer at the wound site caused by the fluid loss through the wound in the exudative stage.
Although currently available dressing materials possess features that contribute to the control of heavy exudate drainage, most also possess significant limitations that retard the overall healing process. For example, thin film dressings such as those described in U.S. Pat. No. 3,645,835, maintain excessive moisture over the wound bed, contributing to the overhydration (maceration) of surrounding skin. Although sponges and gauze support tissue, they require frequent changing, and cause irritation to the wound bed during body movement and dressing removal. Calcium alginates turn into a gelatinous mass during interaction with moisture, are difficult to remove completely, and often dehydrate the wound bed due to the hydroscopic nature of the matrix.
Importantly, none of the presently available devices significantly contribute to or support the autolytic debridement phase, which is the natural removal process of necrotic tissue and debris from the wound. Autolytic debridement is a key early stage event that precedes repair phases of healing. When wound conditions are not optimal for supporting autolytic debridement, then clinical procedures such as surgical removal, irrigation, scrubbing, and enzymatic or chemical methods must be used to remove the necrotic tissue and escar that can inhibit wound healing.
Temporary or permanent wound dressings that are designed to enhance wound healing are needed to cover large open wounds on patients with extensive burns, lacerations and skin damage. Furthermore the ability to produce wound dressings in a variety of shapes to accommodate multiple sizes and forms of injuries is important in the manufacture of useful medical products.
In addition, there continues to be a need for a wound dressing that possesses high moisture absorption capacity, a high rate of absorption, as well as a capacity to regulate moisture at the wound bed-dressing interface. Desirably, such a wound dressing device should stimulate the autolytic debridement process, especially during the heavy exudating phase of wound care management.